1. Field of the Invention
This invention relates to methods and apparatus for reducing the phase-noise in the output from an oscillator, in particular the additive phase-noise. It has one application in frequency-modulated (FM) oscillators and can also provide a non-modulated (CW) output having reduced additive phase-noise.
2. Discussion of Prior Art
Phase-noise can be classified into two main sources. The first is "additive" noise, in which the thermal noise of the oscillator, within the latter's spectral band, is added to its spectrum after amplification in accordance with the loop transfer function of the oscillator. This noise is subsequently converted indirectly to phase-modulation sidebands of the instantaneous oscillator frequency by the removal of amplitude-modulation (AM) components as a result of amplitude-limiting processes within the oscillator. This noise is thermal over the entire band, and is thus uncorrelated with itself over this band (except for a component produced by the above limiting processes, which is unaffected by the present invention).
The second source of phase-noise is "multiplicative" noise, which involves low-frequency noise (typically flicker noise) well outside the spectral band of the loop transfer function but generated within the oscillator. This low-frequency noise is converted upwards to lie within the loop band, as a result of non-linearities within the oscillator. Since, in effect, this noise modulates the oscillator, it produced sidebands corresponding to direct frequency/phase modulation of the oscillator. If the oscillator is itself deliberately modulated, e.g. by VCO (voltage-controlled oscillator) action, these noise sidebands are produced about each deliberately produced modulation sideband. These separate noise sidebands are thus correlated with each other unlike the sidebands from any separate additive noise which, as described above, are uncorrelated over the entire band.
In the case of FM oscillators, the very mechanism of frequency modulation which produces the sidebands tends to degrade frequency stabilities and thus to introduce spurious phase and amplitude noise arising from both additive and multiplicative sources. As will be shown, existing methods of reducing such phase-noise have a limited effect, and it is an object of the present invention to produce a further reduction. This reduction is effected in the additive phase-noise only, not in the multiplicative phase-noise (nor, as stated, in the unaffected additive component mentioned earlier, which effectively becomes multiplicative phase-noise).